You would see this quite often in Visual Studio (mainly 2008) while working with ASP.Net and User Controls. This is been very tricky. It is not actually a problem, but Visiual Studio could not render this control inside another page. It was kind of a pain not being able to see User control inside main pages. It took a while to figure it out.

If you are using HtmlGeneric controls (such as <body>, <div>, <span>, <p>, <link> etc) in your User Control they must have ‘ID’ attribute defined though you are not using it.